docs: def Lin_Kwok(x, mul, mug):
      r'''Calculates a suggested definition for liquid-gas two-phase flow 
      viscosity in internal pipe flow according to the form in [1]_ and shown
      in [2]_ and [3]_.
          
      .. math::
          \mu_m = \frac{\mu_l \mu_g}{\mu_g + x^{1.4}(\mu_l - \mu_g)}
          
      Parameters
      ----------
      x : float
          Quality of the gas-liquid flow, [-]
      mul : float
          Viscosity of liquid, [Pa*s]
      mug : float
          Viscosity of gas, [Pa*s]
  
      Returns
      -------
      mu_lg : float
          Liquid-gas viscosity (**a suggested definition, potentially useful
          for empirical work only!**) [Pa*s]
  
      Notes
      -----
      This model converges to the liquid or gas viscosity as the quality
      approaches either limits.
      
      Examples
      --------
      >>> Lin_Kwok(x=0.4, mul=1E-3, mug=1E-5)
      3.515119398126066e-05
  
      References
      ----------
      .. [1] Lin, S., C. C. K. Kwok, R. -Y. Li, Z. -H. Chen, and Z. -Y. Chen. 
         "Local Frictional Pressure Drop during Vaporization of R-12 through 
         Capillary Tubes." International Journal of Multiphase Flow 17, no. 1 
         (January 1, 1991): 95-102. doi:10.1016/0301-9322(91)90072-B.
      .. [2] Awad, M. M., and Y. S. Muzychka. "Effective Property Models for 
         Homogeneous Two-Phase Flows." Experimental Thermal and Fluid Science 33,
         no. 1 (October 1, 2008): 106-13. 
      '''
      return mul*mug/(mug + x**1.4*(mul - mug))